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By Authority Of 


Legally Binding Document 

By the Authority Vested By Part 5 of the United States Code § 552(a) and 
Part 1 of the Code of Regulations § 51 the attached document has been duly 
INCORPORATED BY REFERENCE and shall be considered legally 
binding upon all citizens and residents of the United States of America. 
HEED THIS NOTICE : Criminal penalties may apply for noncompliance. 

Document Name: APHA Method 2130: Standard Methods for the 

Examination of Water and Wastewater 

CFR Section(s) : 40 CFR 141.121 

Standards Body: American Public Health Association 


For the 

Examination of 
Water and 

: mm 


Prepared and published jointly by: 

American Public Health Association 

American Water Works Association 

Water Environment Federation 

joint Editorial Board 

Arnold E. Greenberg, APHA, Chairman 

Lenore S. Qesceri, WEF 

Andrew D. Eaton, AWWA 

Managing Editor 
Mary Ann H. Franson 

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Copyright ° 1971 by 

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American Water Works Association 
Water Pollution Control Federation 

Copyright® 1976 by 
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American Water Works Association 
Water Pollution Control Federation 

Copyright © 1 981 by 
American Public Health Association 
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Copyright & 1985 by 
American Public Health Association 
American Water Works Association 
Water Pollution Control Federation 

Copyright © 1989 by 
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Water Pollution Control Federation 

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Water Environment Federation 

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The Library of Congress has catalogued this work as follows: 
American Public Health Association. 

Standard methods for the examination of water and wastewater. 
ISBN 0-87553-207-1 

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2130 A. Introduction 

1. Sources and Significance 

Clarity of water is important in producing products destined 
for human consumption and in many manufacturing uses. Bev- 
erage producers, food processors, and treatment plants drawing 
on a surface water supply commonly rely on coagulation, settling, 
and filtration to insure an acceptable product. The clarity of a 
natural body of water is a major determinant of the condition 
and productivity of that system. 

Turbidity in water is caused by suspended matter, such as clay, 
silt, finely divided organic and inorganic matter, soluble colored 
organic compounds, and plankton and other microscopic organ- 
isms. Turbidity is an expression of the optical property that causes 
light to be scattered and absorbed rather than transmitted in 
straight lines through the sample. Correlation of turbidity with 
the weight concentration of suspended matter is difficult because 
the size, shape, and refractive index of the particulates also affect 
the light-scattering properties of the suspension. Optically black 
particles, such as those of activated carbon, may absorb light 
and effectively increase turbidity measurements. 

2. Selection of Method 

Historically, the standard method for determination of tur- 
bidity has been based on the Jackson candle turbidimeter 1 ; how- 
ever, the lowest turbidity value that can be measured directly on 
this instrument is 25 units. Because turbidities of treated water 

Approved by Standard Methods Committee. 1988. 

usually fall within the range of to 1 unit, indirect secondary 
methods also were developed to estimate turbidity. Unfortu- 
nately, no instrument could duplicate the results obtained on the 
Jackson candle turbidimeter for all samples. Because of funda- 
mental differences in optical systems, the results obtained with 
different types of secondary instruments frequently do not check 
closely with one another, even though the instruments are pre- 
calibrated against the candle turbidimeter. 

Most commercial turbidimeters available for measuring low 
turbidities give comparatively good indications of the intensity 
of light scattered in one particular direction, predominantly at 
right angles to the incident light. These nephelometers are un- 
affected relatively by small changes in design parameters and 
therefore are specified as the standard instrument for measure- 
ment of low turbidities. Nonstandard turbidimeters, such as for- 
ward-scattering devices, are more sensitive than nephelometers 
to the presence of larger particles and are useful for process 

A further cause of discrepancies in turbidity analysis is the use 
of suspensions of different types of particulate matter for the 
preparation of instrumental calibration curves. Like water sam- 
ples, prepared suspensions have different optical properties de- 
pending on the particle size distributions, shapes, and refractive 
indices. A standard reference suspension having reproducible 
light-scattering properties is specified for nephelometer calibra- 

Because there is no direct relationship between the intensity 
of light scattered at a 90° angle and Jackson candle turbidity, 
there is no valid basis for the practice of calibrating a nephelom- 
eter in terms of candle units. To avoid misinterpretation, report 

TURBIDITY (2130)/Nephelometric Method 


the results from nephelometric measurements as nephelometric 
turbidity units (NTU). 

Its precision, sensitivity, and applicability over a wide turbidity 
range make the nephelometric method preferable to visual meth- 
ods. The Jackson candle method has been eliminated from the 
1 7th and subsequent editions of Standard Methods. 

3. Storage of Sample 

Determine turbidity on the day the sample is taken. If longer 
storage is unavoidable, store samples in the dark for up to 24 h. 

Do not store for long periods because irreversible changes in 
turbidity may occur. Vigorously shake all samples before ex- 

4. References 


Association & Water Pollution Control Federation. 1985. 
Standard Methods for the Examination of Water and Wastewater, 
16th ed. American Public Health Assoc. Washington, D.C. 

2130 B. Nephelometric Method 

1. General Discussion 

a. Principle: This method is based on a comparison of the 
intensity of light scattered by the sample under defined conditions 
with the intensity of light scattered by a standard reference sus- 
pension under the same conditions. The higher the intensity of 
scattered light, the higher the turbidity. Formazin polymer is 
used as the reference turbidity standard suspension. It is easy to 
prepare and is more reproducible in its light-scattering properties 
than clay or turbid natural water. The turbidity of a specified 
concentration of formazin suspension is defined as 40 nephelo- 
metric units. This suspension has an approximate turbidity of 40 
Jackson units when measured on the candle turbidimeter; there- 
fore, nephelometric turbidity units based on the formazin prep- 
aration will approximate units derived from the candle turbidi- 
meter but will not be identical to them. 

b. Interference: Turbidity can be determined for any water 
sample that is free of debris and rapidly settling coarse sediments. 
Dirty glassware, the presence of air bubbles, and the effects of 
vibrations that disturb the surface visibility of the sample will 
give false results. "True colore 1 that is, water color due to dis- 
solved substances that absorb light, causes measured turbidities 
to be low. This effect usually is not significant in the case of 
treated water. 

2. Apparatus 

a. Turbidimeter consisting of a nephelometer with a light source 
for illuminating the sample and one or more photoelectric de- 
tectors with a readout device to indicate intensity of light scat- 
tered at 90° to the path of incident light. Use a turbidimeter 
designed so that little stray light reaches the detector in the 
absence of turbidity and free from significant drift after a short 
warm up period. The sensitivity of the instrument should permit 
detecting turbidity differences of 0.02 NTU or less in waters 
having turbidity of less than 1 NTU with a range from to 40 
N T U . S e ve ra I r a n ge s a r e ne ce s s a r y to o b ta i n b o t h a d e q u a te cov- 
erage and sufficient sensitivity for low turbidities. 

Differences in turbidimeter design will cause differences in 
measured values for turbidity even though the same suspension 
is used for calibration. To minimize such differences, observe 
the following design criteria: 

I) Light source— Tungsten-filament lamp operated at a color 
temperature between 2200 and 3000°K. 

2) Distance traversed by incident light and scattered light within 
the sample tube— Total not to exceed 10 cm. 

3) Angle of light acceptance by detector— Centered at 90° to 
the incident light path and not to exceed ±30° from 90°. The 
detector, and filter system if used, shall have a spectral peak 
response between 400 and 600 nm. 

b. Sample tubes, clear colorless glass. Keep tubes scrupulously 
clean, both inside and out, and discard when they become scratched 
or etched. Never handle them where the light strikes them. Use 
tubes with sufficient extra length, or with a protective case, so 
that they may be handled properly. Fill tubes with samples and 
standards that have been agitated thoroughly and allow sufficient 
time for bubbles to escape. 

3. Reagents 

cl Turbidity-free water: Turbidity-free water is difficult to ob- 
tain. The following method is satisfactory for measuring turbidity 
as low as 0.02 NTU. 

Pass distilled water through a membrane filter having preci- 
sion-sized holes of 0.2 [xm; ;: the usual membrane filter used for 
bacteriological examinations is not satisfactory. Rinse collecting 
flask at least twice with filtered water and discard the next 200 

Some commercial bottled demineralized waters are nearly par- 
ticle-free. These may be used when their turbidity is lower than 
can be achieved in the laboratory. Dilute samples to a turbidity 
not less than 1 with distilled water. 

/>. Stock turbidity suspension: 

1) Solution I— Dissolve 1.000 g hydrazine sulfate (Caution: 
Carcinogen; avoid inhalation, ingestion, and skin contact.), 
(NIL) ? TLSO.,, in distilled water and dilute to 100 mL in a vol- 
umetric flask. 

2) Solution II— -Dissolve 10.00 g hexamethylenetetramine, 
(CH 2 ) ft N„ in distilled water and dilute to 100 mL in a volumetric 

3) In a 100-mL volumetric flask, mix 5.0 mL Solution I and 
5.0 mL Solution II. Let stand 24 h at 25 ± 3°C, dilute to mark, 
and mix. The turbidity of this suspension is 400 NTU. 

4) Prepare solutions and suspensions monthly. 

* Nuclepore Corporation. 7035 Commerce Circle. Plcasanton. Calif., or equiva 



c. Standard turbidity suspension: Dilute 10.00 mL stock tur- 
bidity suspension to 100 mL with turbidity-free water. Prepare 
daily. The turbidity of this suspension is defined as 40 NTU. 

d. Alternate standards: As an alternative to preparing and di- 
luting formazin, use commercially available standards such as 
styrene divinylbenzene beadst if they are demonstrated to be 
equivalent to freshly prepared formazin. 

e. Dilute turbidity standards: Dilute portions of standard tur- 
bidity suspension with turbidity-free water as required. Prepare 

4. Procedure 

a. Turbidimeter calibration: Follow the manufacturer's oper- 
ating instructions. In the absence of aprecalibrated scale, prepare 
calibration curves for each range of the instrument. Check ac- 
curacy of any supplied calibration scales on a precalibrated in- 
strument by using appropriate standards. Run at least one stand- 
ard in each instrument range to be used. Make certain that 
turbidimeter gives stable readings in all sensitivity ranges used. 
High turbidities determined by direct measurement are likely to 
differ appreciably from those determined by the dilution tech- 
nique, 11 Ac. 

b. Measurement of turbidities less than 40 NTU: Thoroughly 
shake sample. Wait until air bubbles disappear and pour sample 
into turbidimeter tube. When possible, pour shaken sample into 
turbidimeter tube and immerse it in an ultrasonic bath for 1 to 
2 s, causing complete bubble release. Read turbidity directly from 
instrument scale or from appropriate calibration curve. 

c. Measurement of turbidities above 40 NTU: Dilute sample 
with one or more volumes of turbidity-free water until turbidity 
falls between 30 and 40 NTU. Compute turbidity of original 
sample from turbidity of diluted sample and the dilution factor. 
For example, if five volumes of turbidity-free water were added 
to one volume of sample and the diluted sample showed a tur- 
bidity of 30 NTU, then the turbidity of the original sample was 
180 NTU. 

d. Calibrate continuous turbidity monitors for low turbidities 
by determining turbidity of the water entering or leaving them, 
using a laboratory-model turbidimeter. When this is not possible, 
use an appropriate dilute turbidity standard, 1! 3e. For turbidities 
above 40 NTU use undiluted stock solution. 

5. Calculation 

6. Interpretation of Results 

a. Report turbidity readings as follows: 

Nephelometric turbidity units (NTU) 

A x (B + C) 


A — NTU found in diluted sample. 

B — volume of dilution water, mL, and 

C = sample volume taken ('or dilution, mL. 

Report to the 

Turbidity Range 


















t AMCO-AEPA-1 Standard. Advanced Polymer Systems, 3696 C Haven Ave. 
Redwood Citv, Calif. 

b. For comparison of water treatment efficiencies estimate 
turbidity more closely than is specified above. Uncertainties and 
discrepancies in turbidity measurements make it unlikely that 
two or more laboratories will duplicate results on the same sam- 
ple more closely than specified. 

7. Bibliography 

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American Public Health Association. 1901. Report of Committee 
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Wells, P. V. 1922. Turbidimetry of water. J. Amer. Water Works Assoc. 

Baylis, J.R. 1926. Turbidimeter for accurate measurement of low tur- 
bidities. Ind. Eng.Chem. 18:311. 

Wells, P. V. 1927. The present status of turbidity measurements. Chem. 
Rev. 3:331. 

Baylis, J.R. 1933. Turbidity determinations. Water Works Sewage 80:125. 

Rose, H.E. & H.B. Lloyd. 1946. On the measurement of the size 
characteristics of powders by photo-extinction methods. ./. Soc. Chem. 
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Rose, H.E. & C.C.J. French. 1948. On the extinction coefficient: Par- 
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Gillett, T.R., P.F. Meads & A.L. Holven. 1949. Measuring color 
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Jullander, I. 1949. A simple method for the measurement of turbidity. 
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Rose, FI.E. 1950. Powder-size measurement by a combination of the 
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Rose, H.E. 1950. The design and use of photoextinction sedimentom- 
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ODOR (21 50)/lntroduction 


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